In order to perform the conversion between XmlObject and raw xml, the converter
uses a XmlContentConverter implementation. There is no default one, and it must
be cofigured for this converter to funtion using CompassEnvironment.Converter.XmlContent#TYPE).

{@link XmlContentConverter} implementations are wrapped by one of the three built in strategies:
{@link PrototypeXmlContentConverterWrapper}, {@link PoolXmlContentConverterWrapper},
{@link SingletonXmlContentConverterWrapper}, or a user provided fully qualified class name.

handleNulls

Should the converter handle nulls? Handling nulls means should the
converter process nulls or not. Usually the converter will not
persist null values, but sometimes it might be needed
(MarshallingContext.handleNulls()).

Extracted to a method so special converters can control null handling.